The present invention relates to a method of separating oxygen from an oxygen containing gas with the use of an ceramic membrane unit. More particularly, the present invention relates to such a method in which the oxygen containing gas is compressed by a compressor powered by the expansion of a cooled process stream made up at least in part by a retentate formed in the ceramic membrane unit. Even more particularly, the present invention relates to such a method in which the expansion of the process stream is carried out in stages with interstage heating.
Oxygen transport membranes have demonstrated an ability to separate high-purity oxygen from an oxygen containing stream with a purity of at least about 99% and with an oxygen recovery of about 60%. Such oxygen transport membranes are formed from a ceramic that is capable of transporting oxygen ions when both heated to a suitable operational temperature and the opposite sides of the membrane are subjected to an oxygen partial pressure differential. The oxygen ions are formed by oxygen atoms in an oxygen containing feed gaining two electrons at one surface of the membrane. The oxygen is reconstituted at the opposite surface of the membrane by the loss of the electrons from the oxygen ions thus to complete the separation of the oxygen from the feed. Typically, multiple oxygen transport membranes are housed in a ceramic membrane unit that functions to separate oxygen from the oxygen containing feed to produce both an oxygen permeate from the separated oxygen and a retentate from the feed after the separation of oxygen therefrom.
Suitable oxygen transport membrane materials are known as either mixed conducting or ionic. Mixed conducting materials conduct both the oxygen ions and the electrons that are formed upon reconstitution of elemental oxygen from the oxygen ions. Ceramic mixed conducting materials include but are not limited to perovskites. Ionic materials conduct only oxygen ions and thus require an external electric circuit for the return of the electrons. Common materials used in an ionic membrane include, but are not limited to, Yttrium Stabilized Zirconia.
In order to compress the oxygen containing feed, for instance, air, the feed is compressed in a compressor that is powered at least in part by the work extracted from a turboexpander. Typically, a retentate stream, composed of a retentate formed upon separation of the oxygen within the ceramic membrane unit, is expanded in the turboexpander. An example of this is shown in U.S. Pat. No. 5,516,359 in which feed air is compressed and then heated in a direct-fired burner. The resultant heated feed gas is introduced into the ceramic membrane unit to separate oxygen from the feed. The retentate is heated by another direct fired burner prior to its introduction into the turboexpander. The turboexpander is used to power the compressor.
U.S. Pat. No. 5,643,354 discloses an integrated process in which oxygen is recovered from an oxygen-containing feed gas and subsequently is consumed in a coal gasifier. The hot oxygen product exiting the ceramic membrane unit is cooled through indirect heat exchange with water and an expander is used to recover the work needed to drive the feed gas compressor.
xe2x80x9cIon Transport Membrane Technology for Oxygen Separation and Syngas Productionxe2x80x9d, 134 Solid State Ionics, Dyer et al. pp 21-33 (2000), discloses a process in which a hot, low-pressure oxygen product gas, produced by a ceramic membrane unit, is cooled by indirect heat exchange with a cooling medium. After being cooled, the oxygen is compressed to a final delivery pressure. Fuel is used both to heat the feed gas to the desired inlet temperature of the oxygen transport membranes contained in the ceramic membrane unit and to heat the non permeate (i.e., retentate) to the desired inlet temperature to the expander. The work recovered from the expander is used to drive both the feed gas compressor and an oxygen product blower or compressor.
xe2x80x9cAdvanced Oxygen Separation Membranesxe2x80x9d, Report No. TDA-GRI-90/0303, Wright et al., The Gas Research Institute, pp 33-61 (1990), illustrates various schemes for integrating ceramic membrane units with electrical generation systems. In one such integration, feed air is compressed and heated by combustion supported by oxygen contained in a retentate stream that is produced in a ceramic membrane unit. The retentate stream is then fed into a turboexpander that is used to drive a feed air compressor.
U.S. Pat. No. 5,753,007 discloses a process for oxygen recovery from an oxygen-containing feed gas by the use of a ceramic membrane unit in which the retentate stream is cooled and then expanded to recover useful work. In this patent, the degree of cooling is sufficiently high that the work can be extracted for the use of processes that are less energetic than those in which electrical power also is generated. The feed gas can be heated through indirect heat exchange with both the retentate and oxygen product streams. Additionally, the feed stream may be heated further by a combustor interposed prior to the ceramic membrane unit.
An important consideration in the fabrication of any equipment that is used to separate oxygen is its cost. The cost of acquiring a turboexpander increases with its operating temperature due to the use of more exotic and/or more expensive materials. It therefore would be desirable from the standpoint of cost to be able to utilize a turboexpander at a lower temperature, for instance, preferably in a range of between about 300xc2x0 C. and about 650xc2x0 C. However, as the inlet temperature to the turboexpander decreases, there is less energy that can be extracted from a stream to be expanded and, therefore, less energy that is available to drive the feed air compressor. The energy able to be extracted from a stream sufficiently cooled to allow the use of turboexpanders designed to operate at low temperatures can be less than that required to operate the feed air compressor.
As will be discussed, the present invention provides a method of separating oxygen from an oxygen containing feed that is particularly applicable to the use of temperature limited turboexpander components and that can generate sufficient energy from the turboexpansion to drive the feed air compressor as well as other components. Other advantages will become apparent from the following discussion.
A method of separating oxygen from an oxygen containing gas is provided in which a feed stream containing the oxygen containing gas is compressed to produce a compressed feed stream. The compressed feed stream is heated. A ceramic membrane unit also is heated to an operational temperature. Oxygen is separated from the compressed feed stream within the ceramic membrane unit to produce both a retentate that contains residual components of the feed stream and an oxygen permeate formed by the separated oxygen. A process stream composed of at least a portion of the residual components of the retentate is cooled to a temperature below the operational temperature of the ceramic membrane unit. The process stream is expanded with the performance of work in an initial stage of expansion. An expansion stage, as described herein, is comprised of all system components that may be utilized to recover work from an inlet stream. Initial and subsequent stages of expansion are separated by a separate reheating step in which the expanded stream is reheated prior to entering the next stage of expansion. The process stream, after the initial stage of expansion, is reheated, then expanded with the performance of work in a subsequent stage of expansion.
The work of expansion produced by the initial stage of expansion is insufficient to meet the power requirements for the compression of the feed stream, and a sum of the work of expansion of the initial and subsequent expansion stages is at least sufficient to meet the power requirements for the compression of the feed stream. At least a part of a sum of the work of expansion of the initial and subsequent stages of expansion is applied to the compression of the feed stream. An oxygen product stream is extracted from the ceramic membrane unit that is composed of the oxygen permeate.
By having more than one stage of expansion with interstage reheating, sufficient energy can be recovered to power the compressor and, as will be discussed, additional accessories such as product and fuel compressors. At the same time, since the process stream is cooled, less expensive, temperature limited expanders can be utilized. Additionally, the method of the present invention also allows the temperature of the ceramic membranes of the ceramic membrane unit to be set independently of the temperature of the stream to be expanded for power recovery. This can be important when specific low operating temperatures are required for the longevity of the material used in the ceramic membrane.
In accordance with an additional aspect of the present invention, the residual components contained within the retentate include residual oxygen; and the process stream is formed by extracting a retentate stream from the ceramic membrane unit, introducing a fuel stream into the retentate stream, and combusting the fuel in the presence of the residual oxygen contained within the retentate stream. The compressed feed stream is heated through indirect heat exchange with the process stream, thereby cooling the process stream. The compressed feed stream, after having been heated, is introduced into the ceramic membrane unit to separate part of the oxygen contained within the compressed feed stream, thereby heating the ceramic membrane unit to the operational temperature. Thus, in this aspect of the present invention, the energy for heating the feed stream, and therefore the ceramic membrane unit, to an operational temperature is through indirect heat exchange with a process stream formed by combusting a fuel in the presence of residual oxygen in the retentate.
In another aspect of the present invention, the process stream is formed from a retentate stream composed of the retentate and extracted from the ceramic membrane unit. The compressed feed stream is heated through indirect heat exchange with the process stream, thereby cooling the process stream. A heated feed stream is formed by introducing a fuel stream into the compressed feed stream, after having been heated, and combusting the fuel in the presence of part of the oxygen contained within the compressed feed stream. The heated feed stream is introduced into the ceramic membrane unit to separate a remaining part of the oxygen contained within the compressed feed stream, thereby forming the oxygen permeate and heating the ceramic membrane unit to its operational temperature.
In yet another aspect of the present invention, the residual components include residual oxygen, and the compressed feed stream is heated through indirect heat exchange with a retentate stream extracted from the ceramic membrane unit. The compressed feed stream, after having been heated, is divided into first and second subsidiary streams. The process stream is formed by combining the retentate stream with the first subsidiary stream and a fuel stream and combusting the fuel stream in the presence of oxygen contained in said retentate stream and the first subsidiary stream. The second subsidiary stream is heated indirectly from the combustion of the fuel stream, thereby cooling said process stream. The second subsidiary stream is introduced into the ceramic membrane unit to separate the oxygen contained therein to form the oxygen permeate and to heat said ceramic membrane unit to its operational temperature.
It is to be noted that the operational temperature can be in a range from between about 600xc2x0 C. and about 1200xc2x0 C. The temperature below said operational temperature to which said process stream is cooled is preferably in a range of between about 300xc2x0 C. and about 650xc2x0 C. Further, after said initial stage of expansion, the process stream is reheated preferably to a reheated temperature in a range of between about 350xc2x0 C. and about 650xc2x0 C.
In any of the foregoing aspects of the present invention, the expansion exhaust stream can be reheated through indirect heat exchange with the oxygen product stream. Additionally, when the sum of the work of expansion is in excess of that required to compress the feed stream, the sum of said work of expansion is applied additionally to compression of the fuel and oxygen product streams. The expansion exhaust stream is reheated preferably through indirect heat exchange with the oxygen product stream. The ceramic membrane unit can be purged with either a reactive or non-reactive purge stream to increase the separation of the oxygen.
The use of a reactive purge also can be used in connection with a method of the present invention that is designed to produce a nitrogen product. In such a method, a retentate stream is extracted from the ceramic membrane unit and introduced into a further ceramic membrane unit. A reactive purge stream is introduced into the further ceramic membrane unit to separate further oxygen from the retentate stream and thereby to produce a further retentate and further oxygen permeate. In accordance with such a method, the process stream is formed from said further retentate. The compressed feed stream is heated through indirect heat exchange with the process stream and/or a further oxygen permeate stream, composed of the further oxygen permeate. The compressed feed stream, after having been heated, is introduced into the ceramic membrane unit to separate the oxygen therefrom and to heat the ceramic membrane unit to its operational temperature. A nitrogen product stream is formed at least in part from a subsequent expansion exhaust stream that is produced from the subsequent expansion. The expansion exhaust stream can be reheated through indirect heat exchange with both the product stream and the further oxygen permeate stream after its having exchanged heat with the compressed feed stream.
In the aforementioned aspect of the present invention, when the sum of said work of expansion is in excess of that required to compress the feed stream, the subsequent expansion exhaust stream can be divided into first and second nitrogen containing streams. The first nitrogen containing stream is used to form the nitrogen product stream, and the reactive purge stream is formed from a fuel stream and said second nitrogen containing stream. The reactive purge and oxygen product streams are compressed, and the sum of said work of expansion is applied additionally to compression of the reactive purge and oxygen product streams.
In accordance with another method of the present invention that can be used to produce a nitrogen product, the compressed feed stream is heated through indirect heat exchange with the process stream. A heated feed stream is formed by introducing a fuel stream into the compressed feed stream, after having been heated, and the fuel is combusted in the presence of part of the oxygen contained within the compressed feed stream. The heated feed stream is introduced into the ceramic membrane unit to separate a remaining part of the oxygen contained within the compressed feed stream and to heat the ceramic membrane unit to its operational temperature. A retentate stream is extracted from the ceramic membrane unit and is introduced into a further ceramic membrane unit. A reactive purge stream is introduced into the further ceramic membrane unit to separate further oxygen from the retentate stream and thereby to produce a further retentate and further oxygen permeate. The process stream is formed from the further retentate, and a nitrogen product stream is formed at least in part from a subsequent expansion exhaust stream produced from the subsequent expansion. The process stream can be reheated through indirect heat exchange with the product stream and the further oxygen permeate stream.
With respect to the foregoing aspect of the present invention, the sum of said work of expansion is in excess of that required to compress the feed stream. A first nitrogen containing stream, composed of part of the process stream, forms the nitrogen product stream. The reactive purge stream is formed from a further fuel stream and a second nitrogen containing stream composed of a further part of the process stream. The fuel and oxygen product streams are compressed and the sum of the work of expansion is additionally applied to compression of said fuel and oxygen product streams.
A method of the present invention can be adapted for use of a low pressure burner. In accordance with such adaptation, the process stream is formed from a retentate stream composed of the retentate and extracted from the ceramic membrane unit. The compressed feed stream is heated through indirect heat exchange with heat generated through combustion of a fuel and through indirect heat exchange with the process stream, thereby cooling said process stream. The process stream is reheated through indirect heat exchange with a flue gas stream composed of flue gas produced from the combustion of the fuel and the oxygen product stream. The compressed feed stream can be heated through further indirect heat exchange with the heat generated through combustion of the fuel, so that the compressed feed stream initially undergoes the indirect heat exchange with the process stream and subsequently undergoes the indirect heat exchange with heat generated through combustion of the fuel, prior to the further indirect heat exchange. The ceramic membrane unit optionally may be subjected to a purge with steam to increase the oxygen separation. The process stream and an auxiliary air stream can be preheated through indirect heat exchange with the heat generated through the combustion of the fuel. The streams can then be used to support the combustion of the fuel.
It is to be noted, that the term, xe2x80x9cceramic membrane unitxe2x80x9d as used herein and in the claims means any type of reactor that can be used to separate oxygen from an oxygen containing stream and that utilizes oxygen transport membranes, either mixed conducting or ionic, for such separation.